Then I added the capacitors at the floppy power connector, and I lost the right channel! I removed those caps and still no right channel... swap the old op-amp back and still no sound at right channel!

What should I do now?

I got the ADC chips and it sounded great before right channel died tho

It is highly unlikely that screwing up the caps at the floppy power connector would result in the loss of only one channel of audio, as they are just power caps for everything on the card. It is more likely that you've messed up the coupling caps or something else in the signal path of the left/right opamp.

But I didn't touch the coupling caps yet... just op-amp, the op-amp power caps, and added main power cap. What can I do troubleshoot?

Oh and it's not software because I tried to uninstall the driver and reinstall, and did the in-software test where is says "left-channel" then "right-channel". Also not the speakers because I tried them with my MP3 player and it works.

Am I the only one who found out that this soundcard produces a verry annoying sin wave if you try to play 40KHz sine wave through it? Just run Coolpro or Adobe Audition, set your soundcard to 24 bit 192KHz, start a new audio file in coolpro/audition at 32bit 192khz, generate a sin wave starting from 20KHz and ending up at 90KHz, at -1dB, connect headphones on soundcard output and hit play, and listen.. as the main frequency rises, a byproduct frequency produced by the card lowers in frequency... this cannot be seen in normal music because it doesn't contain a so high frequency component, but it makes me wonder, is it because of the dac? is it because of the output opamp? can someone who replaced the output opamp can confirm improvement? or am i the only one who can get this result? I own a creative xtreme gamer and it doesn't do this, and also a elite pro and it doesn't do this either... Does the Xonar D2 have this problem also? what about the essence? I found out about this when i was trying to measure a mic's frequency response, and i played a sweep from 1Hz to 90KHz that lasted 10 minutes using the xonar dx through sennheiser hd 650 and in front of the senn was a mic connected to a e-mu0404usb on a different computer recording the sound and the inverse frequency generated by the xonar can be both heared and seen in the recording... If anybody confirms, then the asus xonar DX should not be labeled 192KHz soundcard.

Did the power caps with 1200uf Nichicons and was feeling brave so I tried the coupling caps. Man those buggers are hard to get off. I tore off a pad! The trace leading to the pad lifted though - I scraped it gently, tinned it then soldered the positive leg of the new cap to it. With a magnifying glass I couldn't even tell for sure if it had made a connection, but I could see a slight disturbance in the tinning on the cap lead. Turned out to have made contact adequately! These mods brightened the sound significantly and sharpened the bass response (gave it more control or whatever. The notes are noticeably more well defined than before.) Thanks for the project!!